"Russia, along with Iran, provides (Syrian President) Bashar al-Assad's regime with weapons and ammunition, and supports it politically, therefore the citizens of these countries present legitimate targets for militants in Syria," Haitham al-Maleh said in an interview with Al Jazeera.

The Geneva Convention, in his interpretation, allows civilians who collaborate with one of the opposing sides to be targeted. At the same time, the 80-year-old politician admitted that rebel commanders in Syria rarely coordinate their actions with the political opposition abroad.

Al-Maleh, who had several spells as a political prisoner in Syria since the 1960s, has received numerous awards for defending human rights in Syria and opposing violence against civilians.

The Syrian rebels have already threatened to target the embassies of Ukraine and Russia and their citizens.

"Let not a single Russian, Ukrainian or Iranian come out of Syria alive," the rebels said in a video broadcast by Ukraine's Ukraina TV channel.

The Russian foreign ministry has confirmed media reports that two Russians and an Italian had been abducted near the western Syrian port city of Latakia as they were travelling along the Tartus-Homs highway.

All three worked for the Hmisho steel company's plant near Homs, the ministry said, adding that the company had received a telephone demand from unidentified individuals for payment as a condition for their release.